Pension and Post-Retirement Benefits - Net Pension and Post-Retirement Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 322	$ 149	$ 601	$ 444
Interest cost	1,158	571	2,136	1,454
Expected return on plan assets	(1,491)	(757)	(2,734)	(2,088)
Net amortization and deferral	631	255	1,264	948
Net pension benefit costs	620	218	1,267	758
Post-retirement Benefits
Defined Benefit Plan Disclosure
Service cost	27	33	54	65
Interest cost	188	191	376	383
Expected return on plan assets	0	(1)	(1)	(1)
Net amortization and deferral	0	53	0	107
Net pension benefit costs	$ 215	$ 276	$ 429	$ 554